I think there is a strong dichotomy between Appolonia and Kay. One represents an old way of life, the life of old Sicily and the Cosa Nostra. While Kay represents the new world, civilization and legitamacy and righteousness. I think that Kay is literally Michael's better half. By choosing Kay he is choosing in essence and making a concious decision to have someone with him who will encourage him to drop a way of life that is negative. This by extension would most likely indicate some underlying intention on his part to want to go legitamite.